This application a 35 USC 371 application of PCT/DE 00/02551 filed on Aug. 2, 2000.
1. Field of the Invention
The invention is based on a fuel injection system for an internal combustion engine and particularly to such a system in which fuel can be injected into a combustion chamber at two different pressures.
2. Description of the Prior Art
An injection system of the type with which this invention is concerned has been disclosed for instance by European Patent Disclosure EP 0 711 914 A1.
For better comprehension of the ensuing description, several terms used herein will first be defined in detail: In a pressure-controlled fuel injection system, by means of the fuel pressure prevailing in the nozzle chamber of an injector, a valve body (such as a nozzle needle) is opened counter to the action of a closing force, and the injection opening is thus uncovered for an injection of the fuel. The pressure at which fuel emerges from the nozzle chamber into the cylinder is called the injection pressure, while the term system pressure is understood to mean the pressure at which fuel is kept available or stored in the injection system. The term stroke-controlled fuel injection system is understood within the scope of the invention to mean that the opening and closing of the injection opening of an injector is done with the aid of a displaceable valve member on the basis of the hydraulic cooperation of the fuel pressures in a nozzle chamber and in a control chamber. Furthermore, an assembly is called central when it is provided jointly for all the cylinders, and local when it is provided for only a single cylinder.
In the pressure-controlled fuel injection system known from EP 0 711 914 A1, with the aid of a high-pressure pump, fuel is compressed to a first high fuel pressure of about 1200 bar and stored in a first pressure reservoir. The fuel at high pressure is also pumped into a second pressure reservoir, in which by regulation of its fuel delivery by means of a 2/2-way valve, a second high fuel pressure of about 400 bar is maintained. Via a valve control unit, either the lower or the higher fuel pressure is carried into the nozzle chamber of an injector. There, by the pressure, a spring-loaded valve body is lifted from its valve seat, so that fuel can emerge from the nozzle opening.
A disadvantage of this known fuel injection system is that first all the fuel has to be compressed to the higher pressure level, and then some of the fuel has to be relieved to the lower pressure level again. Furthermore, since the high-pressure pump is driven by the engine camshaft, it is constantly in operation, even if the desired pressure in the applicable pressure reservoir has already been built up. This constant generation of high pressure and later relief to the low pressure level worsen the efficiency.
According to the invention, it is proposed that a higher pressure level be generated by means of a central pressure booster unit. The pressure booster unit, since it is independent of the camshaft, can be triggered in a targeted way on demand, and as a result the high pressure can be better regulated in terms of quantity. Since the pressure booster unit is not constantly in operation, the losses from friction are reduced accordingly as well.
If the high-pressure side and the low-pressure side of the central pressure booster unit are hydraulically decoupled from one another, then different fuels can be used for the two sides, such as oil for the low-pressure side and gasoline or Diesel fuel for the high-pressure side.